Any Opentherm Gateway worthy of the name should be compatible with an Opentherm boiler, unless you have something special in mind when you say "compatible". The boiler may not be able to report some specific pieces of information, like the water pressure, or DHW flow rate. But all the basic controls should work.
The OTGW I made primarily interacts with the thermostat, so the compatibility on that front is a greater challenge. That's why I did create a
thermostat compatibility topic, while I didn't consider it necessary to do the same for boilers.
If you want to use the gateway without a thermostat, you must transfer all of the thermostat's intelligence into the domotica system. I'm not sure how feasible that is with Loxone. But if you can get Loxone to produce the necessary parameters (like Control setpoint, CH-enable, Max Relative Modulation level), you can feed them to the gateway and it will forward them to the boiler.
Just don't expect to be able to simply send a room temperature and a setpoint to the gateway and then your house magically gets heated. The gateway needs a thermostat to perform that function. Also beware that if your domotica system loses the connection with the gateway or crashes after it set a control setpoint of 70 degrees or so, your house may get quite toasty.